International countries have pledged more than $US2.4 billion in humanitarian aid for the victims of the war in Syria, at a donors conference sponsored by the United Nations.

Delegates from nearly 70 countries and 24 international organisations attended the one-day event chaired by UN Secretary General Ban Ki-moon.

Last month, the UN launched an appeal for $6.5 billion after estimates that the conflict has reversed development gains in Syria by 35 years.

"Half of the total population of Syrian people - nearly 9.3 million individuals - urgently need humanitarian aid," Mr Ban said.

"I am especially concerned about reports of starvation in Syria."

More than 130,000 people have been killed in the three-year conflict and more than three million people have fled the country.

According to aid agencies, more than 10 million Syrians do not have enough food, more than a million children under five are suffering from acute or severe malnutrition, half the population has no access to adequate water sources or sanitation and more than eight million have insufficient access to healthcare.

The host country, Kuwait, led the donations with a pledge for $US500 million.

US Secretary of State John Kerry announced a supplementary aid package of $380 million, bringing the total US humanitarian aid to Syria to $1.7 billion.

"I am proud that the United States is the leading donor of humanitarian aid," Mr Kerry said.

"Syria's civil war is not simply Syria's problem."

Pledges of support from neighbours in the region

Oil-rich Saudi Arabia and neighbouring energy-rich Qatar will both give $60 million. Both countries are strong backers of the rebellion against the regime of President Bashar al-Assad.